1. Field of the Invention
The present invention relates to an alternating-current (AC) motor control apparatus.
2. Discussion of the Background
As a method for estimating the position and speed of an AC motor, a method in which the position and speed of a motor are estimated using detected values or estimated values of induced voltages of the motor, and a method in which the position and speed of a motor are estimated using inductance characteristics (a magnetic saliency) that depend on a magnetic-pole position of the motor have been proposed. A method with which the position and speed of a motor can be estimated over a range from an extremely low speed including a zero speed to a high speed is the latter estimation method. More particularly, in a case in which position control is performed, it is necessary to determine the position of a motor in a state in which the motor halts at a predetermined rotational position, i.e., at the zero speed at which induced voltages become zero. Accordingly, in this case, the latter estimation method, with which the position and speed of a motor can be estimated even when induced voltages are zero, is suitable.
Generally, in the latter estimation method, high-frequency test signals are applied to a motor, and a magnetic-pole position is estimated using detected values of voltages or currents that are generated by applying the test signals. The estimation method has a problem that noise occurs in a frequency band of the test signals, and a problem that responsiveness in estimation of the position and the speed of the motor becomes low because of utilization of filters which extract currents or voltages having a frequency in the frequency band of the test signals or the like.
For example, in Japanese Unexamined Patent Application Publication No. 2001-169590, a control apparatus for controlling, using pulse with modulation (PWM) signals that are synchronized with carrier waves, voltages to be applied to a motor is disclosed, and a scheme for estimating the position of a rotor of the motor by detecting motor currents in synchronization with the carrier waves is proposed. In this scheme, the voltages that are to be applied as high-frequency test signals are changed every period that is half of the period of the carrier waves. The motor currents that are generated by applying the voltages are detected, and a current differential vector is determined every period that is half of the period of the carrier waves. Next, the current differential vector is obtained twice, and the difference between the obtained two current differential vectors (hereinafter, referred to as a “current difference differential vector”) is computed. The difference between two voltage vectors of the voltages to be applied (hereinafter, referred to as a “voltage differential vector”), which corresponds to the current difference differential vector, is computed. The voltages to be applied are controlled so that the phase difference between the current difference differential vector and the voltage differential vector is made to be zero. When the phase difference is made to be zero, the phase of the voltage differential vector is directed to a magnetic-pole position. Accordingly, the voltages to be applied are controlled so that the phase difference is made to be zero.